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Soft matter keywords

Abstract from the original paper

Water drops deposited on hydrophobic materials decorated with dilute micro-posts generally form pearls. Owing to the hydrophobicity of the material, the drop sits on the top of the posts. However, this “fakir state” is often metastable: if the drop impales inside the texture, its
surface energy is lowered. Here we discuss the transition between these two states, considering the drop size as a parameter for inducing this transition: remarkably, it is found that a drop impales when it becomes small, which is interpreted by considering its curvature. This interpretation allows us to propose different recipes for avoiding this detrimental effect.

Soft matter example

In this paper, the authors used hydrophobic surface with regular arrays of micropillars for studying the transition between "fakir state" and "impaled state" as a function of the drop size. They fabricated samples with controlled geometry and coated with a fluropolymer. The contact angle for water on a flat surface was between 100-110° while it was ~160° for a surface with texture.

Fig. 1. Evaporation of a water drop sitting on a hydrophobic surface decorated with pillars (visible in the photos) of diameter d = 3 μm, height h = 4.8 μm and distance l = 17 μm. The initial radius is 120 μm, and the time interval between two successive photos is 5 s. As the drop radius reaches 75 μm (fifth snapshot), the state of the drop abruptly changes, from a super-hydrophobic state to a hydrophilic-like one.

Figure 1 shows the images of a water drop evaporating on a sample with the micropillars of diameter (d) = 3um, height (h) = 4.8 um and mutual distance (l) = 17 um. During a first stage (first five snapshots), the contact angle remained constant as the drop evaporated at a receding angle of 138±3° and the advancing one of 165±2°. However, when the drop becomes smaller than a critical radius R* of 75 um, the angle became much smaller by about 80° with time scale of 1 ms and it continued decreasing: the liquid was very efficiently pinned and the receding angle gradually vanished to zero even though the material was hydrophobic. The authors argued that this phenomenon could be resulted from an impalement of the drop in the texture and they confirmed their interpretation by examining the trace left on the material after evaporation as shown in Fig 2. As shown in Fig. 2., a circular stain was observed at the bottom state of the solid, where it followed the pillars where pinning happened, indicating the drop sank insdie the texture [1].

Fig. 2. Trace left after drop evaporation in the experiment of Fig. 1. A stain resulting from the evaporation-driven flow of dust present in water is clearly visible at the bottom stage of the textured material. The stain follows the pillars, showing that the drop sank inside the texture, and then got pinned on the microstructures. The bar indicates 50 μm.

Then, the authors studied the mechanism and the conditions for such a transition. They did geometrical analysis by considering the curvature of the liquid/vapor pockets below the drop, which gave the critical radius (R*)~l2/h.

For small pillars (h<<l), the critical radius R*~l2/h becomes large as the case of Fig. 1. By putting the dimensions of the geometry, the equation above predicts R* = 60 um, which was comparable to the actual value from Fig. 1 (R* = 75 um). From this relation, we can also consider a way to build a super-hydrophobic material resisting a pinning transition by reducing the size of the microstructures. Having microstructures on the order of 100 nm, we can have a critical radii of the same order. This might be related to the existence of sub-structures at this scale in nature such as plant surfaces or some mosquito's eyes, which remains dry [2].

Fig. 3. Evaporation of a water drop (initial radius of 90 μm) sitting on a hydrophobic surface decorated with pillars of diameter d = 3 μm, height h = 36.5 μm and mutual distance l = 17 μm. The time interval between two successive photos is 9 s. In the second snapshot, the drop radius is about 75 μm, value at which impalement took place in Fig. 1: owing to the use of long pillars, the drop can here reach a much smaller radius without sinking in the texture.

Another way of avoiding impalement of the drop is to make the structure higher while keeping the distance l between posts large enough to maintain a strong hydrophobicity. If the impaled state has lower energy than the fakir state, once the drop touches the bottom surface, it should propagate and this contact can be started by the bending of the interfaces as the drop became smaller. Thus, by having long enough pillars, we can avoid this transition. Figure 3 shows the succesive states of an evaporating drop deposited on a sbustrate of the same pillar density as the Figs. 1 and 2, but with much higher pillars (36.5 um instead of 4.8 um. It was observed that the drop remained at the top of the pillars.

This paper was quite interesting to me because the authors studied the effects of surface geometry, which we can have good control by using microfabrication techniques, on the transition from Cassie to Wenzel states providing possible ways to avoid this transition as well as giving insights about underlying design schemes of some creatures in nature.